Conventionally, a computing system providing a large-scale data storage service to host computers exists. This computing system is known as a storage system or a computer system comprising host computers, a storage apparatus to which the host computers are connected, and a management device of the storage apparatus.
The storage apparatus manages multiple hard disks by the RAID (Redundant Array of Independent/Inexpensive Disks) method. The storage apparatus then logically configures physical storage areas included in the multiple hard disks as a RAID group and provides these as logical volumes to the host computers. The host computers access the logical volumes and request the reading or writing of data to the storage apparatus.
The storage apparatus, by allocating multiple pages to the logical volumes, maintains the data for which the write access is made from the host computer to the logical volumes in the storage areas of the storage device. A page is a logical unit to which a certain degree of storage capacity is given as a result of the physical storage area of the storage device being divided.
As one of the logical configuring technologies of this type, a system in which the storage capacity of the logical volumes is virtualized is known as thin provisioning (e.g., U.S. Pat. No. 6,823,442).
This system sets a logical volume as a virtual volume having a virtualized storage capacity and, each time a write request for the block address of the virtual volume is received from the host computer, allocates pages to the block address for which the write request is made, and changes the actual storage capacity of the virtual volume dynamically.
A group comprising actual storage areas for allocating storage areas to the virtual volume is called a pool and, in this pool, multiple logical volumes of a RAID group called pool volumes exist. Pages are allocated to the virtual volume from the pool volumes.
Though the storage apparatus intends to select pages from the multiple pool volumes equally, with the passage of a long time, a problem arises in that this equality is damaged, which deteriorates the response performance in response to the I/O from the host computer. Therefore, Japanese Unexamined Patent Application Publication No. 2008-234158 discloses the migration of pages among multiple pool volumes.